1. Field of the Invention
The present invention lies in the field of techniques for integrating at least one distributed feedback (DFB) semiconductor laser and a passive strip waveguide.
2. Description of the Prior Art
A method for integrating a DFB laser coupled with a passive strip waveguide on a substrate is disclosed in IEEE Journal of Quan. Electronics, Vol. QE-13, No. 4, April 1977, pp. 220-223. By this method, in a first epitaxy step, an n-Ga.sub.1-x Al.sub.x As layer forming the more deeply disposed layer (for example, having x=0.3 and a thickness of 2 .mu.m), a p-GaAs layer forming the laser-active layer (for example, having a thickness of 0.2 .mu.m), a p-Ga.sub.1-y Al.sub.y As layer (for example, having y=0.2 and a thickness of 0.1 .mu.m), and a p-Ga.sub.1-z Al.sub.z As layer forming the uppermost layer (for example, having z=0.07 and a thickness of 0.2 .mu.m), are successively grown on an n-GaAs substrate by means of conventional liquid phase epitaxy. A third order grating is produced on the uppermost layer of this layer stack by means of chemical etching with the assistance of a mask which has been produced by the use of holographic photolithography. With the exception of the region of the DFB laser, the layer stack is then chemically etched away down to the substrate, thus producing a step extending down into the substrate which separates the region of the laser-active layer from the region of the passive strip waveguide.
Two further layers, that is a p-Ga.sub.1-x Al.sub.x As layer (for example, having x=0.3 and a thickness of 2 .mu.m), and an undoped Ga.sub.1-w Al.sub.w As layer (for example, having w=0.1 and a thickness of 2 .mu.m), are then grown on the stepped surface in the second epitaxy step by means of liquid phase epitaxy under relatively fast growth conditions. The p-Ga.sub.1-x Al.sub.x As layer, which is developed as the first of the two layers, is grown within a time span of 90 seconds at 700.degree. C. with a cooling rate of 5.degree. C./minute. Under these conditions, the p-Ga.sub.1-x Al.sub.x As layer splits or cracks in the step allowing the laser output power to be effectively conducted to the undoped Ga.sub.1-w Al.sub.w As layer, thus forming the passive strip waveguide.
The long-side boundaries of the laser-active region and of the passive strip waveguide region are produced by etching the crystal away down to the substrate. The coupling between the DFB laser and the passive strip waveguide ensues by means of an end face coupling.
As may be seen from FIGS. 2 and 3 on page 221 of the above cited publication, the location of the splitting or cracking of the first layer grown during the second epitaxy step is critical because the split must be formed as precisely as possible at the location of the laser-active layer. The position of this split can only be controlled via the growth conditions utilized during the second epitaxy step.
It is a problem in the art to provide a method for integrating a DFB laser with a passive strip waveguide on a substrate so that DFB lasers coupled with passive strip waveguides can be reproducibly manufactured resulting in a high yield. Such a method needs to guarantee the exact alignment of the DFB laser and the passive strip waveguide relative to one another.